1. Field of the Invention
The present invention relates to semiconductor device structures with thin layers of conductive materials that are oxidation resistant and that act as oxidation barriers to protect underlying conductive or semiconductive structures. More specifically, the present invention relates to semiconductor device structures including with thin layers with noble metals that have been doped to prevent the passage of oxidants therethrough, as well as to methods for forming such thin, doped noble metal layers. The invention also pertains to the use of electroless plating techniques to form oxidation barrier layers from noble metal alloys.
2. State of the Art
The thicknesses of conductive layers and conductive lines in conventional semiconductor devices may themselves prevent significant oxidation of these conductive layers and lines, as well as the passage of oxidants through these layers or lines. In the state of the art, however, the dimensions of features, including the thicknesses thereof, are ever decreasing.
For example, in stacked capacitor structures, the thicknesses of the electrodes and capacitor dielectrics are continuously becoming smaller. As a result of the decrease in capacitor dielectric layer thicknesses, materials with higher dielectric constants, such as tantalum pentoxide (Ta2O5) and barium strontium titanate (BST or BaSrTiO3), are being used with increased frequency.
As another result of the decreasing thicknesses of conductive features of semiconductor device structures, the oxidation of these conductive features that may occur upon formation of adjacent insulative structures often has detrimental effects on the electrical properties of the conductive features. This is particularly true in the thin bottom electrodes of state of the art capacitors as capacitor dielectric layers are formed thereover.
Accordingly, it is desirable to form at least the bottom electrode of a capacitor structure from a material that will not oxidize as a capacitor dielectric layer is being formed thereover or from a material that will substantially retain its conductive properties upon being oxidized. Examples of such materials that have been used as the electrodes in capacitors include platinum (Pt), ruthenium (Ru), ruthenium oxide (RuO2), rhodium (Rh), rhodium oxide (RhO2), iridium (Ir), iridium oxide (IrO2), palladium (Pd), and molybdenum oxide (MoO2).
In addition, due to the ever-decreasing dimensions of features of semiconductor devices, such as the bottom electrodes of capacitors, oxidants are able to more easily travel through these features and to oxidize underlying conductive or semiconductive structures, such as active device regions or conductive (e.g., polysilicon) plugs. In state of the art capacitor structures, this is true even if oxidation resistant materials or materials that form conductive oxides are used to fabricate bottom electrodes.
The problem of oxidants permeating and traveling through the bottom electrode of a capacitor structure is further exacerbated by the extremely high temperatures (e.g., about 600xc2x0 C. and greater) that are employed to form the dielectric layers of state of the art capacitors and anneal these dielectric layers to the underlying bottom electrode. These high temperatures increase the tendency of oxidants to pass through the underlying layer of conductive material.
It has been found that the incorporation of small amounts of beryllium in platinum films retards oxygen diffusion by influencing the grain structure of the platinum film. (See, Roland Stumpf, et al., Retardation of O Diffusion Through Polycrystalline Pt by Be Doping, The American Physical Society, Jun. 15, 1999, at 16 047-16 052 (hereinafter xe2x80x9cStumpfxe2x80x9d)). The platinum film was formed by sputtering, then implanted with beryllium. The use of sputtering to form platinum layers is, however, somewhat undesirable since sputtering may result in layers that do not conformally cover high aspect ratio features, such as the high aspect ratio bottom electrodes that are often present in state of the art, relatively large surface area capacitor structures.
The use of boron-doped platinum films in capacitor structures has also been investigated, but boron doping of platinum was determined not to have as significant an affect on oxygen diffusion as beryllium doping of platinum. Id. at 16,050-51.
Platinum-rhodium and platinum-iridium alloys are also known to prevent the passage of oxidants therethrough. These layers of oxidation barrier alloys have been formed by processes such as reactive radiofrequency (RF) sputtering, which typically require high process temperatures of about 500xc2x0 C. or greater.
The inventor is not aware of any art that teaches the use of electroless plating techniques for forming conductive, oxidation-barrier layers of semiconductor device structures in a single step and that substantially conformally cover high aspect ratio features of semiconductor device structures, while preventing the passage of oxidants therethrough to underlying structures.
The present invention includes a substantially confluent, conductive, oxidation barrier layer or structure that is oxidation resistant and that substantially prevents oxidants from passing therethrough, as well as capacitor structures and other semiconductor device structures including such conductive layers.
An exemplary embodiment of the conductive layer includes platinum doped with phosphorous or boron. For example, the conductive layer may include about 0.1% to about 5% boron, by weight of the layer.
As another example, the conductive layer may include an alloy of noble metals, such as a platinum-rhodium alloy or a platinum-iridium alloy.
The present invention also includes methods for forming conductive layers or structures that resist oxidation and that substantially prevent the passage of oxidants therethrough. By way of example, electroless plating techniques may be used to form both the exemplary doped platinum conductive layer or structure and the noble metal alloy conductive layer or structure. The processes that are used to form the conductive layer are preferably substantially conformal processes, which form the layer on both nonvertical surfaces of underlying structures and substantially vertical surfaces of underlying structures, including the substantially vertical surfaces of structures with high aspect ratios.
As an example of an electroless plating process that may be used to form the conductive oxidation barrier, a substrate may be introduced into an aqueous metal solution including at least one metal salt and at least one reducing agent. When a reducing agent that includes dopant atoms, such as a borohydride, is employed, the metal atoms of the metal salt and the dopant atoms of the reducing agent are said to be xe2x80x9cco-depositedxe2x80x9d upon formation of a barrier layer. Alternatively, when a combination of salts of different metals are used, the metals are said to be co-deposited as an alloy.
Examples of the one or more metal salts that may be used in the aqueous metal solution include, without limitation, salts of noble metals, such as platinum, rhodium, iridium, ruthenium, palladium, or alloys including any of these metals. The one or more reducing agents that may be used in an aqueous metal solution in accordance with the method of the present invention may include, but are not limited to, agents that will result in a conductive layer that is doped with boron or another dopant that is useful for forming a conductive layer that acts as an oxidation barrier. For example, dimethylamineborane (DMAB), potassium borohydride, sodium borohydride, or other borohydrides may be used as the one or more reducing agents of the aqueous metal solution. When the oxidation barrier includes a metal alloy, hydrazine may be used as the reducing agent, as may other suitable reducing agents that cause dopants to be introduced into a formed oxidation barrier layer.
The methods of the present invention may be used to form an oxidation barrier to be positioned adjacent a conductive layer or a conductive layer that also acts as an oxidation barrier. Accordingly, by forming an oxidation barrier in accordance with the inventive method, one or more underlying components of a semiconductor device structure, such as polysilicon plugs or active device regions that underlie the bottom electrodes of capacitor structures, may be protected from oxidation.
The present invention also includes semiconductor device structures, including capacitor structures, that have been formed in accordance with teachings of the present invention, or that include doped metal oxidation barriers or noble metal alloy oxidation barriers.
Other features and advantages of the present invention will become apparent to those of skill in the art through a consideration of the ensuing description, the accompanying drawings, and the appended claims.